Wide frequency band antenna



Feb. 27, 1951 o, wlLLOUGHBY 1 I 2,543,085 V WIDE FREQUENCY BAND ANTENNAFiled April 15, 1945 2 sheets-sheet 1 Feb. 27, 1951 E. o. WILLOUGHBY Q2,543,085

WIDE FREQUENCY BAND ANTENNA Filed April 13, 1945 v 2 Sheets-Sheet 2Patented Feb. 27, 1951 UNITED STAT-ES PATENT OFFICE WIDE nastiness-11mANTENNA Eric Osborne Willoughby, London, England, as-

signor, by mesne assignments, to International StandardElectric'Corporation, New York, N. Y., a corporation: of DelawareApplication al.151945 Serial No.: 588,269 In. Great. Britain April. 21',1944,

6 Claims. 1

The present invention relates to radio antennas, and concernsparticularly antennas of small size adapted for stream lining for use onaeroplanes.

It is frequently found that antennas used-on aeroplanes are liable to bebroken off' by the air resistance when driving at very high speed. It istherefore most desirable that the antenna should be as small as possibleso that the minimum air resistance be offered.

The present specification describes a new type of dipole antenna whichcan be. made. very short compared with the wave-length, (for exampleabout one fifth).

According to the invention, there is provideda radio antenna comprisinga. hollow metallic body enclosing two electromagnetically coupledresonating systems. both tuned to the operating frequency; and atransmission line feeder for conveying currents to or from the antennacoupled to one of the said systems.

According to the invention also. there is pr.0. vided' a radio antennacomprising a. hollow resonator, a resonating circuit inside the saidresonator and coupled electromagnetically thereto, and a transmissionline coupled to thesaid circuit,

the said resonator and circuit. being each tuned to the operatingfrequency.

The invention also provides; a radio antennav comprising a hollowcoaxial resonator, aresonating input transmission line inside the saidresonator and coupled thereto,, and a, transmission line feeder forconveying currents. to or from the. antenna coupled to. thesaid inputline, the said.

input line and resonator being. tuned to the-operating. frequency.

The invention willfbe described with referencev to the accompanyingdrawings, in which:

Fig. 1 shows diagrammatically the features of an antenna according tothe. invention;

Figs. 2, 3 and 4 show equivalent. circuits :em

played in explaining the action of the antenna;

Figs. 5 and 6 show respectively a longitudinal sectional View, and an.end view (with the cap removed), of an antenna. constructed. accordingto the invention; and

Figs. 7 and; 8. show diagrams corresponding to part. of Fig. 1 to showthe manner in which a rectifier may be connected inside the antenna.

Fig. .1 shows diagrammatically an. antenna according to the invention.It comprises-arodor wire. conductor I loaded. at the ends with relatively large metal masses land 3.. Surrounding the. conductor I. is. ahollow cylindrical-s bodys 4 forming therewith, a hollow resonator.consisting.

of.l an. inner, coaxial transmission line open at both ends, which endsare terminated by the capacities between, the body 4 and the masses 2and 3. These terminating capacities may be supplemented by adjustabletuning capacities 5 and 6.

Currents are led toor'from the'antennaby a coaxial transmis'sion linefeeder I entering the body 4 at the median plane l2, and the centralconductor 8 is connected to a point. on a conductor 9 placed inside thebody 4 parallel to the rod I. The conductor 9 is connected at H)" to thewall of the body 4 (either directly or. through an appropriate blockingcondenser, not shown) and the other end is terminated by a tuningcondenser H.

The conductor 9' forms with the body 4 a tuned input transmission linewhose electromagnetic field is coupled with that of the resonatorformed" by the conductor I and the body- 4. The system is equivalenttotwo coupled resonant circuits both of which should be tuned tothefrequency of the waves to be radiated or received. According to theusual practice with coupled tuned circuits, the Q value (ratio ofreactance to resistance) of the two circuits should be the same and theyshould be sufiic'iently' over coupled (that is, the

coupling factor should be a little greater than- The red I'- with thetwo masses 2 and 3 forms a dipole antenna, but its potential variationsare transferred to the outer surface of the enclosing body 4 which thusacts as the real radiator. The physical length of the rod 1 and body 4can be small compared with the wave length ('for example about one fifthof the wave length). The inner transmission line will have a voltagenode at the median plane l2 and a current antinode, andthe current atanyinstant will be nearly the same at all points of the body owing to itsshort length.

Under these conditions the short inner coaxial transmission, line.formed by. the rod 1 and. th body 4' as seen from the open ends .issubstantially equivalent. to. the network of lumped reactances shown inFig. 2. Let 2d be the total length of. the. body 4 and let 0=2.1rd/where A- isthe wave-1ength, then it-is easil-y'shown that thereactanceofeach. of the equivalent oon-- densers K is. iZ1 eat 0, andthe reactance of the inductance 2! is 721 sin 20,, in which Z1 is thecharacteristic impedance of. the; inner transmission line. line beingopen at both ends,. (assuming that the excitation by the conductor 9 isbalanced),- there will. be? a voltage node -at theeentne on: themedianeplane 1.2,..andsoas seen:

from either end, the network of Fig. 2 is in effect short circuited inthe centre as indicated by the dotted line. The inner transmission linetherefore presents an impedance at either end which may be representedby the network of Fig. 3.

The outer surface of the body 4 can be regarded as forming with groundan outer transmission line open at both ends, there being a voltage nodeat the median plane I2. If Z2 is the characteristic impedance of thisouter transmission line, then the reactance presented by this line ateach open end will be 722 tan 0. V

It is now possible to construct an'approximate equivalent circuit forthe antenna of Fig. 1. This is shown in Fig. 4. The small squares 2 and3 represent the end masses 2 and 3 of Fig. 1. These are shown connectedto earth by approximately equal capacities C2 and C3. The mass 2 is alsoconnected to ground through the two open end impedances of the inner andouter transmission lines, which are in series. These are representedrespectively by the network 12, K2, and by the inductance L2 in serieswith a resistance T2 representing the radiation resistance of half theantenna. Similarly the mass 3 is connected to earth throughcorresponding impedances 13, K3, and. L3, T3, where where w is 21r timesthe frequency.

The condensers S2 and S3 are the tuning condensers 5 and 6 shuntedacross the open ends of the inner transmission line.

The exciting conductor 9 is represented in Fig. 4 by two seriesconnected windings coupled respectively with Z2 and la. The excitationis such that the potentials of 2 and 3 are equal and opposite, so "thatthe two halves of Fig. 4 are effectively in series. S2 and S3 will beadjusted so that the series circuit resonates at the oper atingfrequency. As all the elements of the cir cuit are known the effective Qvalue can be determined. For example, if the antenna be supposed to becut inhalf on the median plane I2, then a resonance curve relating tothe impedance looking into the cut end to the frequency can bedetermined for half the antenna, from which the Q value can be found.

By suitable choice ofthe impedance of the transmission line 7 (Fig. l)and the'manner in which the conductor 8 is connected to the conductor 9,the Q of the input circuit may be made to have the same value.

In order that the antenna may be efiicient it is necessary that themasses 2 and 3 should be made relatively large, so that the reactancesof C2 and C3 are reasonably small, otherwise the series circuit of Fig.4 will have such sharp tuning that only a small band width can behandled.

by the antenna.

Figs. 5 and 6 show one form in which an antenna according to Fig. 1 maybe made up. The body 4 has the flattened segmental section indicated inFig. 6. The conductor I of Fig. 1 is represented by two parallel rods IAand iB, the ends of which are seen in Fig. 6. In Fig. 5 only the rod IAis visible.

The ends of the body are closed by insulating plates I3 and I 4. Outsidethese plates are fixed similarly shaped metal plates I5 and IS. Theplates I3, I4, I5 and I6 have clearance slots HA and. "B for therods IAand IB, and the parts 4 are clamped together by means of the nuts I8which screw on the ends of the rods at both ends. The slots permit thespacing of the rods to be adjusted.

The conductor 9 of the input line comprises a flat strip arrangedbetween the rods IA and IB. The strip 9 is bent round at one end andclamped by screws to the body 4 at II]. The conductor 8 of thetransmission line 1 passes through an insulating disc I9 closing the endof the tube and is secured to a metal disc 29 which rests on the discI9. Another insulating disc 2i covers the metal disc 20, and the stripconductor 9 rests on the top of the disc 2i, being held down by thescrew 22 provided with an insulating sleeve and washer, as shown, toprevent the strip 9 from being short circuited.

A small metal disc 23 carried on a screwed shank passing through thewall 4 is arranged below the strip 9 and forms. therewith an adjustablecondenser corresponding to II of Fig. 1.

The plates I5 and I6 are provided with metal tongues 25 and 26 solderedor otherwise electrically secured thereto. through corresponding slotsin the plates I3 and I4. Discs 2! and 28 with screwed shanks similar to23 pass through the walls of the body 4 and leading masses 2 and 3 ofFig. 1.

The transmission line I is provided with a foot or flange 32 by whichthe antenna may be fixed to the underside of a wing of an aeroplane, forexample. The transmission line I should be about a quarter wavelengthlong.

. movements.

.may be stream-lined by means of a suitably The use of two parallel rodsto form the conductor I enables the characteristic impedance of theinner transmission line to be given a suitable value, and the couplingfactor between the inner trnasmission line and the input line may be adjusted by adjusting the spacing of the rods in the slots HA and NB. Itis, of course, not essential to use two rods. Gne of them could beomitted, or more than two could be used.

The section of the body 4 of the antenna is only approximately correctlystream-lined. The sharp forward edge sets up some eddies which assist inthe prevention of ice formation. The end caps may be suitablystream-lined for end-on The co-axial transmission line I By'suitablechoice of the capacitie of these condensers, the impedance of thetransmission line I may be made to load the input line so as to producethe desired value of Q. If a direct connection were made between theconductors 8 and 9, this connection would probably need to beinconveniently near one end of conductor 9.

When the antenna is usedas a receiver, a suitable rectifier may veryconveniently b housed inside the body 4. If a low impedance rectifier"These tongues pass (such as a copper oxide or selenium rectifier) isused, it may be connected as shown diagrammatically in Fig. 7. A plate35 insulated from the body 4 forms a by-pass condenser. The rectifier 33is connected between th conductor 9 and the plate 35 near the end 10 ofthe conductor 9. The conductor 8 is in this case connected to the plate35 and not to the conductor 9, and carries the rectified current, theradio frequency current being by-passed. When a high impedancerectifier, such as a diode, i used, it may be connected as shown in Fig.8. The plate 36 is insulated from the body 4 to form a by-passcondenser, and the conductor 9 is connected to this plate at N. Theconductor 8 is'also connected to the plate 36. The diode 34 is connectedat the other end of the conductor 9 near the condenser l I. therectified current flows to th conductor 8, and the radio frequencycurrent is bypassed at l0.

What is claimed is:

1. A radio antenna of the dipole type with means for rendering saidantenna short compared to the operating frequency comprising a coaxialline type section open at both ends, a

capacitive load coupled across each end of said coaxial section, saidcoaxial section being dimensioned to resonat with said capacitive loadsat the operating frequency, a circuit resonant at the operatingfrequency and coupled to said coaxial section at the center thereof, anda translating device coupled to said resonant circuit.

2. An antenna according to claim 1 in which said resonant circuitcomprises a conductor mounted parallel to and spaced from the innerconductor of said coaxial section and means connecting said conductor ofsaid resonant circuit to the outer conductor or said coaxial section.

Again 3. An antenna according to claim 1 in which said capacitive loadscomprise metallic masses substantially closing the respective ends ofsaid coaxial section and providing relatively large capacities to earth.

4. An antenna according to claim 1 in which said resonant circuitcomprises a conductor inside said coaxial section and parallel to theaxis thereof, said conductor being connected at one of its ends to theouter conductor of said coaxial section and a tuning condenserconnecting said conductor of said resonant circuit at the other of itsnds to the outer conductor of said coaxial section.

5. An antenna according to claim 1 further comprising a capacitypotentiometer connecting said translating device to said resonantcircuit.

6. A radio antenna according to claim 1 further comprising a rectifierconnected across said resonant circuit.

ERIC OSBORNE WILLOUGHBY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,005,779 Gothe June 25, 19352,159,648 Alford May 23, 1939 2,284,405 McArthur May 26, 1942 2,287,220Alford June 23, 1942 2,287,845 Varian June 30, 1942 2,304,377 RobertsDec. 8, 1942 2,344,171 Rote Mar. 14, 1944 2,382,693 Dallenback et a1.Aug. 14, 1945 2,424,089 Gethmann July 15, 1947

